Re: [RFC 0/4] dma-fence: Deadline awareness

From: Rob Clark
Date: Wed Jul 28 2021 - 11:23:30 EST

On Wed, Jul 28, 2021 at 6:57 AM Pekka Paalanen <ppaalanen@xxxxxxxxx> wrote:
> On Wed, 28 Jul 2021 15:31:41 +0200
> Christian König <christian.koenig@xxxxxxx> wrote:
> > Am 28.07.21 um 15:24 schrieb Michel Dänzer:
> > > On 2021-07-28 3:13 p.m., Christian König wrote:
> > >> Am 28.07.21 um 15:08 schrieb Michel Dänzer:
> > >>> On 2021-07-28 1:36 p.m., Christian König wrote:
> > >>>> At least AMD hardware is already capable of flipping frames on GPU events like finishing rendering (or uploading etc).
> > >>>>
> > >>>> By waiting in userspace on the CPU before send the frame to the hardware you are completely killing of such features.
> > >>>>
> > >>>> For composing use cases that makes sense, but certainly not for full screen applications as far as I can see.
> > >>> Even for fullscreen, the current KMS API only allows queuing a single page flip per CRTC, with no way to cancel or otherwise modify it. Therefore, a Wayland compositor has to set a deadline for the next refresh cycle, and when the deadline passes, it has to select the best buffer available for the fullscreen surface. To make sure the flip will not miss the next refresh cycle, the compositor has to pick an idle buffer. If it picks a non-idle buffer, and the pending rendering does not finish in time for vertical blank, the flip will be delayed by at least one refresh cycle, which results in visible stuttering.
> > >>>
> > >>> (Until the deadline passes, the Wayland compositor can't even know if a previously fullscreen surface will still be fullscreen for the next refresh cycle)
> > >> Well then let's extend the KMS API instead of hacking together workarounds in userspace.
> > > That's indeed a possible solution for the fullscreen / direct scanout case.
> > >
> > > Not for the general compositing case though, since a compositor does not want to composite multiple output frames per display refresh cycle, so it has to make sure the one frame hits the target.
> >
> > Yeah, that's true as well.
> >
> > At least as long as nobody invents a mechanism to do this decision on
> > the GPU instead.
> That would mean putting the whole window manager into the GPU.

Hmm, seems like we could come up with a way for a shader to figure out
if a fence has signaled or not on the GPU, and then either sample from
the current or previous window surface?